pgrep: Add command `pgrep` #338

Job	Run time
cargo test (ubuntu-latest)	35s
cargo test (macOS-latest)	23s
cargo clippy -- -D warnings (ubuntu-latest)	18s
Code Coverage (ubuntu-latest, unix)	2m 3s
cargo test (windows-latest)	2m 44s
cargo clippy -- -D warnings (macOS-latest)	18s
cargo check (ubuntu-latest)	16s
Code Coverage (macos-latest, macos)	1m 52s
cargo clippy -- -D warnings (windows-latest)	2m 14s
Code Coverage (windows-latest, windows)	5m 31s
cargo fmt --all -- --check	3s
cargo check (macOS-latest)	16s
cargo check (windows-latest)	2m 7s
	18m 40s

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